Using the lambda, mu, sigma method, the sigma and lambda terms allow the 5th percentile lower limit of normal to be independently calculated throughout the age-height spectrum. Using the lambda, mu, sigma method, the sigma and lambda terms allow the 5th percentile lower limit of normal to be independently calculated throughout the age-height spectrum. For the FEV1/FVC ratio (above) and for both FVC and FEV1 (not shown), the normal range is wider in young children and in older adults. (From Reference 10, with permission.) Bruce H Culver Respir Care 2012;57:136-145 (c) 2012 by Daedalus Enterprises, Inc.